High Efficiency Should Be a Drain: A Closer Look at Condensing Boilers

Posted by Lester Shen | Date October 15, 2013

The market share of condensing boilers, sometimes called high efficiency boilers, has increased dramatically over the last several years, thanks in part to utility programs that provide rebates. Unfortunately, many of these newly installed condensing boilers only achieve a fraction of their expected ideal case energy savings. Senior Mechanical Engineer Russ Landry explains why, and offers solutions to optimize their performance.

When a boiler burns natural gas to heat water, it produces both carbon dioxide and water vapor--which is really just diluted steam. In a conventional boiler all of this steam goes right out the chimney with most of its potential heating energy wasted:

The lost heat up the flue results in an annual fuel utilization efficiency of about 80 percent for conventional boilers. In order to increase boiler efficiency, you need to increase the amount of heat that is transferred to the hot water supplied to the radiators in the building. This will also reduce the temperature of the flue gases exhausting up the chimney. However, reduce the temperature too much, the flue gases will condense, which can lead to rapid corrosion and failure in conventional boilers. For the past 70+ years, boiler system designers and operators have created and maintained conventional boiler systems with operating conditions that prevent condensation.

A high efficiency condensing boiler captures much more of that steam’s energy to heat the water more efficiently. As the steam’s energy is more fully captured, these boilers are designed to safely drain the acidic condensate out of the boiler:

The actual efficiency of a condensing boiler depends on the temperature of water going into the boiler, also called the return or entering water temperature. The lower the return water temperature, the greater the boiler’s ability to condense (and operate most efficiently). Condensing boilers are somewhat more efficient than conventional boilers at any return water temperature. However, their efficiency really starts to jump when the return water drops below about 130°F so that they can actually condense some of the diluted steam that otherwise goes out the chimney. If there isn’t condensate draining out of the boiler’s outlet when it’s been running, then it is only achieving the minimum expected savings at that time.

Reaching the initial point of condensation is a great start, but the opportunities for energy savings increase when the return temperature goes down further and more of the waste steam is condensed to help heat the building. The big differences in optimal operating conditions mean that it’s not enough to simply replace conventional boilers. The sensitivity to return water temperature must drive both retrofit system design and long-term operation in order to get the most savings from replacing conventional boilers with condensing boilers. For example, an older building’s heating system may have originally been designed for 160°F return water temperature, but insulation improvements mean that the boiler water temperature can now be significantly lower—even in the coldest winter weather.

Reducing Boiler Water Flow

A key way to optimize condensing boiler efficiency is to keep the boiler system water flow as low as possible. This saves on electrical pump energy, but the even greater benefit is boiler efficiency improvement. Slowing down the water flow helps reduce the return water temperature, so the boilers can condense more of the otherwise wasted steam and operate more efficiently.

The return water temperature is reduced at lower flow rates because as the water flows more slowly through the heating system, each gallon of water gives up more heat to the building and comes back at a lower temperature. For example, the heat output from one gallon flowing through with a 20°F temperature drop can be equaled by half a gallon flowing through with a 40°F temperature drop. This higher temperature drop can mean a much lower boiler return water temperature—and increased boiler efficiency.

If nothing else is changed, the lower flow rate will lead to a 20°F lower return water temperature, which can mean up to a 4 percentage point higher boiler efficiency. Keeping the flow as low as possible combines with outdoor reset control to achieve maximum savings over the course of the year.

Keeping the flow as low as possible should be considered when selecting a pump at the time of boiler replacement and controlling the pump during on-going operation. Variable speed pumping is the most common way to minimize system flow. However, the savings realized depends on the details of the control of the pump’s speed, the piping arrangement, and the type of valves in the system.

Outdoor Reset Control

In the coldest winter weather, many building heating systems need high boiler temperature with 180°F supply water and 160°F return water. Condensing boiler savings are lowest under these conditions, but an outdoor reset control can automatically lower these temperatures during milder weather. In this way, an outdoor reset control minimizes return water temperature under varying conditions so that the boilers can condense some of the otherwise wasted steam as often as possible. Condensing boiler energy savings increase even further when the boiler system temperature is lowered as far as possible below the point of condensation While each building heating system needs a high enough boiler system temperature to deliver heat under any weather condition, outdoor reset controls can help a building operator optimally balance occupant comfort with optimal boiler performance.

Current CEE Research

Our field study will quantify how operating conditions impact the installed energy savings of condensing boilers and determine the potential for optimization through low-cost upgrades. We are nearly finished with the selection of twelve condensing boiler sites representative of education, government, and apartment buildings. We are choosing sites to represent a variety of common boiler system characteristics and have begun to monitor a number of these sites. CEE researchers are investigating part-load efficiency issues in addition to the critical impact of boiler water temperature on efficiency. We’ll complete monitoring by the end of summer 2014.

* This research supported in part by a grant from the Minnesota Department of Commerce, Division of Energy Resources through the Conservation Applied Research and Development (CARD) program. And with co-funding by CEE in support of its nonprofit mission to advance research, knowledge dissemination, and program design in the field of energy efficiency.